ABSTRACT

Introduction T he major afferent pathways that provide the central nervous system with direct access to information about the external environment are described in Chapter 7. In parallel with these pathways at the posterior fossa and supratentorial levels is another ascending system, the consciousness system. This system extends from the medulla to the cerebral cortex. The consciousness system is a diffuse system that regulates the state of arousal, atten­ tion, and the sleep-wake states and modulates cortical reactivity to stimuli. The pathways o f the consciousness system arise from the brainstem, hypothalamus, and basal forebrain and modulate activity o f the cerebral cortex through projections to the thalamus or directly to the cerebral cortex. In this chapter, the anatomy and physi­ ology o f the consciousness system, its role in the regu­ lation of wakefulness and sleep, and pathologic states of altered consciousness, which are a reflection of deranged activity within the system, are described.